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Engine Oil

Oil is essential to the engine. It provides a lubricating film between all moving parts, and also has a cooling function. Maintaining an adequate supply of clean oil is one of the best ways of making an engine last. Some engine oil is consumed during normal operation, making it necessary to regularly check and "top up" the oil supply. Since oil also becomes contaminated and breaks down over time, regular oil changes are necessary.

Engine oil requirements are defined by the oil's American Petroleum Institute (API) service rating, and by its Society of Automotive Engineers (SAE) viscosity rating. This information can be found on the oil can or bottle, often on a standard label as shown in Fig. 3-1.

Fig. 3-1. Engine oil label showing API service rating (1) and SAE viscosity number (2).

The API service rating designates the type of use for which the oil is suited. It is based on the additives that are used to resist oil break-down and carbon formation, inhibit corrosion, resist foaming, neutralize acids, and help remove deposits and keep contaminants suspended in the oil.

For gasoline engines, the service ratings range from SA to SF, with SF rated for the most demanding applications. For diesel engines, the ratings range from CA to CD, with CD rated for the most severe use. Oils may have service ratings for both gasoline and diesel engines, SF/CC for example.

The SAE viscosity rating indicates resistance to flow. An oil designated SAE 40 has a higher viscosity, greater resistance to flow, than an oil designated SAE 30. While higher viscosity oils will generally offer greater engine protection, they may be too thick and resistant to flow, and may inhibit starting during cold weather.

The correct engine oil viscosity depends on the operating temperature range. See the viscosity vs. temperature tables elsewhere in this section. Select a viscosity rating for the lowest anticipated temperature at which the engine must start.

Multi-viscosity oils have additives that make them suitable for use over a wider range of temperatures. For example, an oil rated SAE 10W-30 offers the flow characteristics of SAE 10 at low temperatures, but the protection capability of SAE 30 at engine operating temperature. The "W" in the SAE rating indicates that the oil is suitable for winter use.

Oils of different viscosity ratings can be mixed, but mixing oils of different API service ratings or brands is not recommended.

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